With the developments of mobile phones and PDAs (Personal Digital Assistants) in recent years, efforts have been made in the art to produce smaller-size and higher-definition display devices. Attention has been drawn to stereoscopic display devices as a new extra value that can be added to mobile devices. Generally, a means for displaying stereoscopic images relies on a process of projecting images having a binocular disparity respectively to the left and right eyes. There is a stereoscopic display device including a display panel which has a lenticular lens or a parallax barrier as an image swapping means. Another stereoscopic display device is of the time division type which includes two light sources for applying light to the right and left eyes to project left and right parallax images to the right and left eyes (see, for example, Patent document 1).
The stereoscopic display devices of the above types are suitable for use on mobile devices in that they do not require the observer to wear special glasses and hence to take the trouble of wearing glasses. Actually, mobile phones incorporating parallax-barrier stereoscopic display devices are available as commercial products (see, for example, Non-patent document 1).
According to the above principles, however, since spatially separate parallax images are projected, the observer can see proper stereoscopic images in a limited area. The area in which the observer can see stereoscopic images is called a stereoscopic viewing area, and is determined when the stereoscopic display device is designed. If the positions of the eyes of the observer are shifted out of the stereoscopic viewing area, then problems arise in that the left image and the right image may look overlapping (so-called dual image) and an image with a reversed protrusion depth (so-called pseudo-stereoscopic image) may be observed.
The stereoscopic viewing area will be described below.
First, a stereoscopic viewing area achieved when a parallax barrier is used as an image swapping means will be described below.
FIG. 1 shows by way of example an optical model wherein parallax images are projected onto the left and right eyes of the observer in a parallax-barrier stereoscopic display device. FIG. 1 is a cross-sectional view as seen from above the head of the observer, showing a positional relationship in which both eyes (right eye 55R and left eye 55L) of the observer are positioned on observational plane 30 which is spaced from the display surface of the display device by optimum observation distance OD and the center between the eyes of the observer is aligned with the center of the display panel.
The display panel (not shown) comprises a group of light modulating elements as a matrix of pixels (e.g., a liquid crystal panel). FIG. 1 shows only pixels at both ends and the center of the display panel among right-eye pixels 4R and left-eye pixels 4L which are alternately arrayed. Parallax barrier 6 which functions as an image swapping means is disposed behind the display panel as viewed from the observer. Parallax barrier 6 is a barrier (light shield plate) with a number of narrow stripe-shaped vertical slits 6a, and is arranged such that its longitudinal direction is perpendicular to the direction along which left-eye pixels 4L and right-eye pixels 4R of the display panel are arrayed. A light source (not shown: so-called backlight) is disposed further behind the parallax barrier. Light emitted from the light source travels through slits 6a, has its intensity modulated by the pixels of the display panel, and is then projected toward the observer. The directions of light projected from right-eye pixels 4R and left-eye pixels 4L are limited by the presence of slits 6a. The paths of light rays which are emitted from slits 6a and which travel through the closest pixels are shown as light rays 20. These light rays 20 define right-eye area 70R where images projected from all right-eye pixels 4R are superposed and left-eye area 70L where images projected from all left-eye pixels 4L are superposed. In right-eye area 70R, the observer can observe only the images projected from all right-eye pixels 4R. In left-eye area 70L, the observer can observe only the images projected from all left-eye pixels 4L. Therefore, when right eye 55R of the observer is positioned in right-eye area 70R and left eye 55L of the observer is positioned in left-eye area 70L, the observer visually recognizes parallax images projected onto the right and left eyes thereof as a stereoscopic image. Stated otherwise, when right eye 55R of the observer is positioned in right-eye area 70R and when left eye 55L of the observer is positioned in left-eye area 70L, the observer can observe a desired stereoscopic image.
The display device shown in FIG. 1 is designed such that all images (width P′) projected from right-eye pixels 4R and left-eye pixels 4L (width P) at the distance OD are superposed in order to maximize width L of right-eye area 70R and left-eye area 70L. Width P′ of the projected images is determined mainly from distance h between slits 6a and the pixels, pixel pitch P, and optimum observation distance OD. If P′ is increased, then the width of right-eye area 70R and left-eye area 70L is also increased, but the stereoscopic viewing area in which the observer can visually recognize stereoscopic images may not necessarily be increased because it is impossible to place the eyes of the observer in any desired positions. If it is assumed that the distance between the eyes is represented by e, then the display device should preferably be designed such that P′ is equal to inter-eye distance e. If P′ is smaller than inter-eye distance e, then the stereoscopic viewing area is limited to P′. If P′ is greater than inter-eye distance e, then it is only that an area in which both eyes are positioned in right-eye area 70R or left-eye area 70L is increased. Minimum distance ND and maximum distance FD up to the display panel, at which the observer can sec stereoscopic images, are also determined by inter-eye distance c, right-eye area 70R, and left-eye area 70L.
As described above, the area in which the observer sees stereoscopic images based on projected parallax images is determined by not only right-eye area 70R and left-eye area 70L which are optically determined by the image swapping means, but also the inter-eye distance e of the observer. Consequently, the stereoscopic viewing area may be expressed by an area around midpoint M between right eye 55R and left eye 55L of the observer.
As shown in FIG. 2, stereoscopic viewing area 71 thus defined is of a diamond-shaped rectangle. However, stereoscopic viewing area 71 shown in FIG. 2 is effective only when the plane including the eyes of the observer and the surface of the display panel lie parallel to each other.
FIG. 3 shows an optical model wherein parallax barrier 6 functioning as the image swapping means is positioned in front of the display panel as viewed from the observer. As is the case with the example in which parallax barrier 6 is positioned behind the display panel, the display device is designed such that the observer is in optimum observation position OD and the images (width P′) projected from the left and right pixels (width P) are superposed. The paths of light rays which are emitted from the pixels and which travel through the closest slits 6a are shown as light rays 20. These light rays 20 define right-eye area 70R where images projected from all right-eye pixels 4R are superposed and left-eye area 70L where images projected from all left-eye pixels 4L are superposed.
FIG. 4 shows a stereoscopic viewing area created by using a lenticular lens as an image swapping means.
FIG. 4 is similar to FIG. 3 except that the image swapping means is different.
An optical model using a lenticular lens with the observer shifted out of the stereoscopic viewing area will be described below.
FIG. 5 is a cross-sectional view as seen from above the head of the observer, showing the observer shifted to the right out of stereoscopic viewing area 71 which is expressed using midpoint M between right eye 55R and left eye 55L. Right eye 55R of the observer is positioned outside of right-eye area 70R, and left eye 55L is positioned within right-eye area 70R. At this time, light rays 20 which are emitted from left-eye pixels 4L and right-eye pixels 4R and which travel through the principal points (vertexes) of closest cylindrical lenses 3a do not reach the position of right eye 55R of the observer. Light rays 21 which are emitted from left-eye pixels 4L and which travel through the principal points (vertexes) of second closest cylindrical lenses 3a, define second left-eye area 72. In FIG. 5, the observer observes the image projected from left-eye pixels 4L with right eye 55R, and observes the image projected from right-eye pixels 4R with left eye 55L. Therefore, when the observer observes parallax images, the protrusion depth is reversed (so-called pseudo-stereoscopic image), and the observer fails to observe a desired stereoscopic image.
To solve the above problem, there has been proposed a process of detecting the position of the observer at all times and switching around the displayed images of right-eye pixels and left-eye pixels depending on the detected position (see, for example, Patent document 2).
There has also been proposed a process of capturing an image of an observer with a camera, detecting a viewpoint position from an obtained image of the face of the observer, and adjusting parallax images (see, for example, Patent document 3).
For detecting a viewpoint position, there has been proposed a process of detecting a pupil with an infrared irradiator and a camera (see, for example, Patent document 4).